Modeling a Ring Network Topology in BlockSim

When designing a control system, a major consideration is the
topology of the computer network that will be used to communicate
between the human-machine interface (HMI) and the controlled devices. There are many topologies in use today, such as point-to-point, bus,
star, ring, mesh, and hybrid networks [1]. In order to select the
best topology for the control system, it is necessary to understand the
benefits and risks associated with each type of network topology. By modeling proposed network configurations in
BlockSim, the analyst
can predict the availability of each topology before the control system
design is finalized.

This two-part article addresses how to use
BlockSim 10 to create a
reliability block diagram (RBD) model of a ring network with
bi-directional communication paths that will provide
information on both the system availability and the availability of
particular communication paths. The first part discusses how to
create the system-level RBD model starting from the ring network
schematic. The subsequent article will demonstrate how to augment
the model to incorporate blocks that represent the state of
communication between the HMI and each connected device.

A simplified
version of a ring network will be studied. The network consists of
the components listed in Table 1. A schematic is shown in Figure 1.

The first step to create an RBD from the schematic is to determine
the success criteria. The system is considered operational when
either HMI 1 or HMI 2 can communicate with either of the I/O boards or
the PLC. Therefore, if at least one of the following scenarios is
true, then the system is operational:

HMI 1 communicates with I/O Board 1

HMI 1 communicates with I/O Board 2

HMI 1 communicates with PLC

HMI 2 communicates with I/O Board 1

HMI 2 communicates with I/O Board 2

HMI 2 communicates with PLC

The second step is to determine what items must operate for each
success criterion. For success criterion 1, I/O Board 1, C 50, ENS
5 and HMI 1 must operate along with either of the following paths:

C 15 and ENS 1 and [C 101 or (C 12 and ENS 2 and C102)]

C 45 and ENS 4 and C 34 and ENS 3 and C 23 and ENS 2 and [C 102 or (C 12
and ENS 1 and C 101)]

For success criterion 2, I/O Board 2, C 30, ENS
3 and HMI 1 must operate along with either of the following paths:

C 23 and ENS 2 and [C 102 or (C 12 and ENS 1 and C 101)]

C 34 and ENS 4 and C 45 and ENS 5 and C 15 and ENS 1 and [C 101 or (C 12
and ENS 2 and C 102)]

For success criterion 3, PLC, C40, ENS 4 and
HMI 1 must operate along with either of the following paths:

For success
criterion 4, I/O Board 1, C 50, ENS 5 and HMI 2 must operate along with
either of the following paths:

C15 and ENS 1 and [C 201 or (C 12 and ENS 2 and C 202)]

C45 and ENS 4 and C 34 and ENS 3 and C 23 and ENS 2 and [C 202 or (C 12
and ENS 1 and C 201)]

For success criterion 5, I/O Board 2, C 30, ENS
3 and HMI 2 must operate along with either of the following paths:

C 23 and ENS 2 and [C 202 or (C 12 and ENS 1 and C 201)]

C 34 and ENS 4 and C 45 and ENS 5 and C 15 and ENS 1 and [C 201 or (C 12
and ENS 2 and C 202)]

For success criterion 6, PLC, C40, ENS 4 and
HMI 2 must operate along with either of the following paths:

C34 and ENS 3 and C 23 and ENS 2 and [C 202 or (C 12 and ENS 1 and C
201)]

C45 and ENS 5
and C 15 and ENS 1 and [C 201 or (C 12 and ENS 2 and C 202)]

The
third step is to create the RBD. Although a diagram could be
constructed directly from the preceding logical statements, the number
of blocks in the diagram can be reduced by combining the statements
prior to creating the diagram. One way to do this is described
next.

By reviewing the logical statements above, it can be seen that
for any device to communicate with either HMI 1 or HMI 2, one of the
following sets of components must be operational:

Clockwise Path: C 15 and ENS 1 and [(C 101 and HMI 1) or (C 12 and ENS 2
and C 102 and HMI 1) or (C 201 and HMI 2) or (C 12 and ENS 2 and C 202
and HMI 2)]

Counterclockwise Path: C23 and ENS 2 and [(C 102 and HMI 1) or (C 12 and
ENS 1 and C 101 and HMI 1) or (C 202 and HMI 2) or (C 12 and ENS 1 and C
201 and HMI 2)]

Now, one logical statement can be written for each
device in terms of the Clockwise Path and Counterclockwise Path above. Therefore, for communication between I/O Board 1 and either HMI 1 or HMI
2:

Conclusion

This article presented the procedure for building a reliability block
diagram to represent a ring network. First, the success criteria
for the system were developed. Then, these criteria were used to
create logic statements that represent the combinations of components
that are necessary for the system to be operational. Finally, the
logical statements were combined and used to create the system-level
RBD.